Question: A running track is the ring formed by two concentric circles. If the circumferences of the two circles differ by $10\pi $ feet, how wide is the track in feet?

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Answer: Call the radius of the outer circle $r_1$ and that of the inner circle $r_2$. The width of the track is $r_1-r_2$. The circumference of a circle is $2\pi$ times the radius, so the difference in circumferences is $2\pi r_1-2\pi r_2=10\pi$ feet. If we divide each side by $2\pi$, we get $r_1-r_2=\boxed{5}$ feet.